Method For Producing One Or More Air Separation Apparatuses, And Equipment For Air Separation By Cryogenic Distillation

ABSTRACT

In a method for producing an apparatus for air separation by cryogenic distillation, at least one gas in the air is obtained as a product by air separation in a cold box including at least one column, the cold box being supplied with air at a first pressure by N air compressors that compress the air from the ambient pressure to the first pressure, the first pressure being greater than 12 bar absolute and less than 35 bar absolute, N being equal to or greater than 3, and the total power of the N compressors being greater than 10 MW, a group of n size classes of a frame for compressors being predefined, n being equal to 2 or 3 or 4 or 5, and the N compressors being selected such that all of the N compressors have a size selected from the n sizes of the group.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2012/052924, filed Dec. 13, 2012, which claims the benefit of FR1162170, filed Dec. 21, 2011, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for producing one or more apparatuses for separating air by cryogenic distillation and to an installation for separating air by cryogenic distillation.

BACKGROUND

In order to limit engineering costs and through repetitiveness allow procurement savings, standardized ranges of air separation apparatuses have been created, ranging up to tonnages of the order of 700 MT/D or even 1000 MT/D. These standardized productions do not always fully correspond exactly to what the customer or customers require in terms of flow rate and/or pressure but in these small units cost is the key factor in optimization and standardization meets this key criterion well.

Above and beyond these capacities, because energy adopts an increasing level of importance, units referred to as modular units have been introduced, the approach this time being toward standardizing certain key sections, but meeting customer requirements as closely as possible and taking the parallel considerations of energy and investment into consideration in the ratings.

EP-A-0504029 describes a pump cycle based on the idea of a single machine with a single large high-pressure air compressor.

This approach allows appreciable savings on investment by comparison with the traditional pump cycle, introducing all the energy required using this single air machine the delivery pressure of which can be between around 12 bara to 35 bara, whatever the demanded production purities and pressures. However, this single machine, when we get up to very high powers, is difficult to produce and has to be started with complex and expensive starter systems on the motors, referred to as regulators. What is more, the number of manufacturers is extremely small and this limits, although without completely removing, the technical-economical benefit of this approach. Some of these problems are described in “Turbomachinery Limitations for Large Air Separation Plants” by Wolentarski, Cryogenic Processes and Equipment Conference, Century 2-Emerging Technology Conferences, San Francisco, Calif., Aug. 19-21, 1980.

On matters of maintenance and reliability, spare parts are purchased for all these critical machines, both for the compressors and for the motors. It is entirely acceptable to have a single set of spare parts for a group of identical machines installed on the same site or even in the same country.

Motor technology varies according to power: indeed above and beyond 25 MW no motors other than synchronous motors exist on the marketplace, since present-day asynchronous motor technology is unable to cross this milestone without taking a very large industrial risk.

The article “Oxygen Plants: 10 years of development and operation” in CEP July 1979 describes the use of synchronous motors and explains that three sizes of synchronous motor are stored to replace Air Liquide Group European compressors in the event of breakdown.

In general, the material cost of an air separation unit with single high pressure high pressure air compressor cycles (excluding storage and vaporization and high-voltage utilities) can be broken down into four main parts:

-   -   i) compression function (compression, motor, starter equipment         and associated electrical equipment): 45% to 50%,     -   ii) cold box and associated function: 30% to 35%,     -   iii) the function of purifying the hot proportion of the air         before it enters the cold box: 10% to 15%;     -   iv) miscellaneous: 5% to 10%.

It is therefore clear that reducing the costs and increasing the reliability of the compressors, the motors and the starter equipment is a matter of some priority.

With methods that employ a cold booster compressor driven by a turbine as described in U.S. Pat. No. 5,475,980 of the type described in EP-0504029, all of the power is introduced by the high-pressure air compressor. If an air booster compressor is present, it is driven by an air turbine or a nitrogen turbine, but not by a steam turbine or a motor. Arrangements on the exchange line, the number and type of turbines coupled to a booster compressor and the distillation columns are the parameters that allow production to be made compatible with the purities, pressures and flow rates demanded by the customer.

SUMMARY OF THE INVENTION

The present invention stems from the fact that, for a customer demanding the supply of a product or products with a given flow rate, a given purity and a given pressure, this deliverable has of necessity to correspond to a power which is not a combination of a given air flow rate and a given high-pressure air pressure.

In order to maintain the benefit of meeting customer requirements as closely as possible, while at the same time standardizing the key part in order to allow repetitiveness savings on this part and volume effect savings with suppliers, and also and above all remaining just within technological, technical or even economical thresholds (where there is an appreciable number of potential suppliers) the number N of high-pressure compressors for producing the air supplied to the cold box of the air separation apparatus that meets the requirements of the customer is between 3 and 10, for example, 3, 4, 5, 6, 7, 8, 9 or 10 compressors in parallel may be used.

A high-pressure compressor compresses air from atmospheric pressure to between 12 and 35 bar absolute.

The N compressors may all have the same frame size, this size preferably being predefined by the manufacturer. Otherwise, at least one of the compressors may be of one size and at least one other may be of another size, the total number of sizes used for compressing the air in the apparatus not exceeding 2 or 3 or 4 or 5.

The compressors are generally built by specialist manufacturers, each manufacturer producing a range of compressors of different predefined models, as described in U.S. Pat. No. 6,116,027. The idea of “frame size” employed by compressor manufacturers covers a combination of components of fixed size, representing one particular compressor model.

The compressor contains moving parts, such as a main impeller, and pinions contained inside a gear case. This case is made up of a single casting which does, however, have removable cover plates providing access to the main parts.

As described in EP-A-1302668, the frame sizes are chosen so that by using a single compressor of defined frame size, chosen between seven different models, it is possible to cover the entire range of flow rates to be compressed.

Likewise, in US-A-20030109948 it is indicated that a single compressor is chosen from 7 different models for each of the 11 air separation apparatuses.

The teaching of the present invention is that by combining at least three compressors, each having a frame size, and by choosing the compressor frame size from just between 2, 3, 4 or 5 sizes, it is possible to cover the entire range of air flow rates to be compressed. Thus, the manufacturer will order certain compressor sizes in large quantities and enjoy numerous advantages, including that of having fewer spare parts to manage.

By combining these 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 compressors with one another in the knowledge that, for each model, and therefore for a given frame size, there is a potential flexibility of the order of 20% on flow rate and 30% on output pressure, all of the necessary powers together, for whatever requirement in terms of product, flow rate, pressure and purity corresponding to a power between around 10 MW and around 150 MW, can be covered by choosing the elements for handling the air downstream of the compression, for example the turbines, booster compressors, exchangers, pumps and distillation columns and by choosing how these are connected with one another, in a way known to those skilled in the art.

For most air separation apparatuses to be constructed in the world or in a given country, a limited number of frame sizes may be used, this number ranging from 2 to 5. The number of sizes may thus be three or four. A larger or smaller number of the same compressor may be used according to the apparatus. This would make it possible to reduce the stocks of spare parts, because the parts for a compressor of an apparatus of one frame size will be usable not only for the other compressors of the same apparatus but also for the compressors of other apparatuses.

By working just within the technological limits of these machines, for example just below 25 MW, only asynchronous motors may be installed, thus allowing savings in terms of reliability, such machines being more robust than synchronous motors.

Because the power is lower in relative terms, direct starts, or even starts using reactance or autotransformers of the motors of these machines can be performed rather than the need to employ the intermediary of regulators or progressive starters (better known as “soft starters”) which are extremely expensive for very high-capacity motors.

Another subject matter of the invention provides a method for producing an apparatus for separating air by cryogenic distillation, in which at least one gas from the air is obtained by way of product by separating air in a cold box comprising at least one column, the cold box being supplied with air at a first pressure by N compressors compressing air from ambient pressure to the first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of the N compressors being greater than 10 MW, a group of n compressor frame size classes being predefined, n being equal to 2 or 3 or 4 or 5, the compressors in each class each having a frame of the same dimensions and the N compressors being chosen so that all the N compressors are of at least one size chosen from the n frame sizes of the group.

According to other subject matters of the invention, there is provided an installation for separating air by cryogenic distillation, comprising at least one cold box containing at least one column for producing at least one gas of the air by air separation, N air compressors, the installation comprising no air booster compressor apart from, potentially, an air booster compressor driven by an air turbine intended for the cold box or for at least one of the cold boxes or by a turbine driven by nitrogen coming from the cold box or from at least one of the cold boxes, and means for sending air from the N compressors to the cold box or to the cold boxes, each of the N compressors being capable of compressing air from ambient pressure to a first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of N compressors being greater than 10 MW, a group of n compressor frame sizes being predefined, n being equal to 2 or 3 or 4 or 5, each frame of a given size having the same dimensions and the N compressors being chosen so that all the N compressors have a frame chosen from the group of n sizes;

-   -   the N compressors have the same frame size.     -   the N compressors do not all have the same frame size.     -   at least a first of the N compressors has a first frame size of         the n sizes and at least a second of the N compressors has a         second of the n sizes.     -   at least a third of the N compressors has a frame of a third of         the n sizes.     -   each compressor compresses the air to the same outlet pressure         from the same inlet pressure which is atmospheric pressure.

The N compressors can be considered to compress the air to one and same pressure if the outlet pressure of a compressor differs from the outlet pressure of another compressor by less than 20% or even less than 10% of the pressure of the flow rate formed by mixing all the compressed flow rates.

-   -   the group of the n compressors is made up of a first frame size         of a compressor for a power of between 5 and 7 MW, of a second         frame size, of a compressor for a power of between 12 and 16 MW,         and of a third frame size of a compressor for a power of between         19 and 25 MW, n being at least equal to 3.     -   n is equal to 2,     -   n is equal to 3,     -   n is equal to 4,     -   n is equal to 5,     -   the separation method does not use an air booster compressor         driven by a motor or a steam turbine,     -   the separation method uses at least one air booster compressor         driven by a turbine driven by air intended for the cold box or a         turbine driven by nitrogen coming from the cold box.

Possibly:

-   -   the installation comprises N air compressors.     -   the installation comprises at least N+1 air compressors.     -   at least one of the air compressors supplying the cold box or         boxes is chosen from outside the group of n models.     -   all the frames air compressors supplying the cold box or cold         boxes are chosen from the group of n sizes.     -   a predefined model of compressor has a predefined frame size.     -   the installation comprises just one single cold box.     -   the installation comprises at least two cold boxes supplied by         at least three air compressors each having a frame chosen from         the group of n sizes.

Another subject matter of the invention provides a method for producing several apparatuses for separating air by cryogenic distillation, in each of which at least one gas from the air is obtained by way of product by separating air in a cold box comprising at least one column, the cold box being supplied with air at a first pressure by N air compressors compressing air from ambient pressure to the first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of the N compressors being greater than 10 MW, a group of n compressor frame size classes being predefined, n being equal to 2 or 3 or 4 or 5, the compressors in each class each having a frame of the same dimensions and therefore of the same size and the N compressors for each of the apparatuses being chosen so that all the N compressors are of one size chosen from the n frame sizes of the group.

The separation apparatuses are preferably not identical.

Each of the separation apparatuses is preferably designed to separate a flow rate of air that differs by at least 10%, or even by at least 25%, from the flow rates of air which are separated in the other apparatuses.

Thus the spare parts for the N compressors of each apparatus are stored in a common place.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides an embodiment of the present invention.

FIG. 3 provides an embodiment of the present invention.

DETAILED DESCRIPTION

Apparatuses according to certain embodiments of the invention will be described in greater detail with reference to the figures which depict schematic drawings.

In FIG. 1, a cold box BF of an air separation apparatus contains a system of columns and an exchanger allowing air to be cooled to the distillation temperature. The air to be distilled 7 has previously been purified in a purification unit E in order to remove the water and carbon dioxide.

The apparatus produces at least one product 9 which may be gaseous oxygen and/or gaseous nitrogen and/or liquid oxygen and/or liquid nitrogen and/or gaseous argon and/or liquid argon.

The air at atmospheric pressure is compressed in three compressors C1, C2, C3. Each of these compressors may, for example, have the same capacity. Each compressor compresses the air to the purification pressure, equal to at least 12 bar abs, preferably less than 35 bar abs. The three flow rates of air 1, 2, 3 compressed in the compressors C1, C2, C3 are combined into a single flow rate 6 and purified together in the unit E.

All of the air sent to the cold box comes from the compressors C1, C2, C3 and the compressors C1, C2, C3 send all of their air to the cold box BF.

Each compressor C1, C2, C3 compresses the air to the same outlet pressure from the same inlet pressure which is atmospheric pressure. However, the three compressors can be considered to compress the air to the same pressure if the output pressure of a compressor differs from the output pressure of another compressor by less than 20%, or even less than 10% of the pressure of the flow rate 6 formed by mixing all the compressed flow rates.

Each compressor C1, C2, C3 is driven by an asynchronous motor M1, M2, M3. Each motor M1, M2, M3 has a respective starter D1, D2, D3, these starters being of the direct (better known as “direct online”), reactance (“reactance”) or autotransformer type. None of these motors is started by a soft starter or a regulator, making the installation simpler.

The cold box, and therefore the three compressors, handle air to produce at least 4000 metric tonnes of oxygen per day. Thus, instances in which each compressor is of the same model, each compressor handles at least 6666 metric tonnes of air per day. The three compressors are driven by motors, preferably at substantially constant speed. In such cases, having defined three frame sizes of compressor as the preferred range, just one of these sizes will be chosen for all three compressors.

Alternatively, the frame of one compressor C1 may be of one size and the frames of the two compressors C2, C3 of another size, the two sizes being chosen from the preferred range that comprises three sizes.

Likewise, the frame of compressor C1 may be one size, the frame of compressor C2 of another and the frame of compressor C3 of a third; in that case, one of each of the three sizes of the preferred range will be ordered.

The preferred range may comprise two, four or five models, rather than three.

At least one air booster compressor may be present. If it is, it is driven by a turbine driven by air intended for the cold box and/or by a turbine driven by nitrogen coming from the cold box, for example a cryogenic turbine, having an inlet temperature of below −50° C. By contrast, no use is made of any air booster compressor driven by a steam turbine or a motor.

It will be readily understood that the invention can be extended to apparatuses having four compressors, five compressors or six compressors in parallel. The specific case of five compressors is illustrated in FIG. 2.

In FIG. 2, a cold box BF of an air separation apparatus contains a system of columns and an exchanger that allows the air to be cooled down to the distillation temperature. The air 7 that is to be distilled has been purified beforehand in a purification unit E so as to remove the water and carbon dioxide.

The apparatus produces at least one product 9 that may be gaseous oxygen and/or gaseous nitrogen and/or liquid oxygen and/or liquid nitrogen and/or gaseous argon and/or liquid argon.

The air at atmospheric pressure is compressed inside five compressors C1, C2, C3, C4, C5 connected in parallel. Each of these compressors preferably has the same capacity. Each compressor compresses the air to the purification pressure equal to at least 12 bar abs, preferably less than 35 bar abs. The five flow rates of air 1, 2, 3, 4, 5 compressed in the compressors C1, C2, C3, C4, C5 are combined into a single flow rate 6 and purified together in the unit E.

All of the air sent to the cold box comes from the compressors C1, C2, C3, C4, C5 and the compressors C1, C2, C3, C4, C5 send all of their air to the cold box BF.

Each compressor C1, C2, C3, C4, C5 compresses the air to the same outlet pressure from the same inlet pressure which is atmospheric pressure. However, the five compressors may be considered to compress the air to the same pressure if the outlet pressure of a compressor differs from the outlet pressure of another compressor by less than 20%, or even less than 10% of the pressure of the flow rate 6 formed by combining all the compressed flow rates.

Each compressor C1, C2, C3, C4, C5 is driven by an asynchronous motor M1, M2, M3, M4, M5. Each motor M1, M2, M3, M4, M5 has a respective starter D1, D2, D3, D4, D5, these starters being of the direct (“direct online”), reactance (“self”) or autotransformer type. None of the motors is started by a soft starter or a regulator, making the installation far far simpler.

The cold box, and therefore the five compressors handle air to form at least 4000 metric tonnes of oxygen per day. Thus, each compressor is of the same model and handles at least 4000 metric tonnes of air per day. The five compressors are driven by motors, preferably at constant speed. In that case, having defined three compressor frame sizes as the preferred range, just one of these sizes will be chosen for all five compressors.

Alternatively, the frame of compressor C1 may be of one model size and the frame of the other four compressors of another size, the two sizes being chosen from the preferred range which comprises two or three sizes.

Likewise, the frame of compressor C1 may be of one size, the frame of compressor C2 of another and the frames of the other compressors of a third; in that case, the three sizes of the preferred range will each be ordered, one compressor in one size, one of the second and three of the third.

The preferred range may comprise four or five sizes rather than three.

Thus, for N compressors and n compressor frame sizes in the preferred range, n being between 2 and 5 and N being greater than or equal to 3 for a given air separation apparatus, at least one compressor will be of one of the n sizes and the N−1 other compressors may be of at least one of the n−1 other sizes.

One possibility is for each of the frames of N compressors to be of the same size, chosen from the n of the preferred range, comprising between 2 and 5 sizes.

If not, the frame of just one of N compressors may be of a first size in the preferred range, and all the others of one of n−1 other sizes, chosen from the preferred range.

For N equal to 3, 4 or 5, each of the frames of the compressors may be of a different size, chosen from the preferred range.

For N equal to or greater than 4, two of the compressor frames may be of a first size and the others of at least one other size, chosen from the n−1 others of the range.

For N equal to or greater than 4, three of the compressor frames may be of a first size and the other or others of at least one other size, chosen from the n−1 others in the range.

At least one air booster compressor may be present. If it is, it is driven by a turbine driven by air intended for the cold box and/or by a turbine driven by nitrogen coming from the cold box. By contrast, no use is made of any air booster compressor driven by a steam turbine or by a motor.

In FIG. 3, two cold boxes BF1, BF2 of an air separation apparatus each contain a system of columns and an exchanger that allows the air to be cooled down to the distillation temperature. The air 7 that is to be distilled has previously been purified in a purification unit E so as to remove the water and carbon dioxide. The two cold boxes may or may not be identical.

Each cold box produces at least one product 9, 9′ which may be gaseous oxygen and/or gaseous nitrogen and/or liquid oxygen and/or liquid nitrogen and/or gaseous argon and/or liquid argon.

The air at atmospheric pressure is compressed in three compressors C1, C2, C3. Each of these compressors may, for example, have the same capacity. Each compressor compresses the air to the purification pressure, equal to at least 12 bar abs, preferably less than 35 bar abs. The three flow rates of air 1, 2, 3 compressed in the compressors C1, C2, C3 are combined into a single flow rate 6 and purified together in the unit E.

All of the air sent to the cold boxes comes from the compressors C1, C2, C3 and the compressors C1, C2, C3 send all their air to the cold boxes BF1, BF2. The compressed air 7 is split into two parts 8, 8′ each of which is sent to one of the cold boxes.

It is also possible to combine all the flow rates 1, 2, 3 and split them into two, each cold box being associated with its own purification unit.

Each compressor C1, C2, C3 compresses the air to the same outlet pressure from the same inlet pressure which is atmospheric pressure. However, the three compressors can be considered to compress the air to the same pressure if the outlet pressure of a compressor differs from the outlet pressure of another compressor by less than 20% or even less than 10% of the pressure of the flow rate 6 formed by combining all the compressed flow rates.

Each compressor C1, C2, C3 is driven by an asynchronous motor M1, M2, M3. Each motor M1, M2, M3 has a respective starter D1, D2, D3, these starters being of the direct (“direct online”), reactance (“reactance”) or autotransformer type. None of the motors is started by a soft starter or a regulator, making the installation simpler.

The cold boxes handle in total at least 4000 metric tonnes of oxygen per day. Thus, in instances in which each compressor has the same frame size, each compressor handles at least 6700 metric tonnes of air per day. The three compressors are driven by motors, preferably at substantially constant speed. In that case, having defined three compressor frame sizes as the preferred range, just one of these sizes will be selected for all three compressors.

At least one air booster compressor may be present. If it is, it is driven by a turbine driven by air intended for at least one of the cold boxes and/or by a turbine driven by nitrogen coming from at least one of the cold boxes. By contrast, no use is made of any air booster compressor driven by a steam turbine or a motor.

Alternatively, compressor C1 may be of one frame size and the two compressors C2, C3 of another frame size, the two sizes being chosen from the preferred range which comprises three sizes.

Likewise, compressor C1 may be of one size, compressor C2 of another and compressor C3 of a third; in that case, one of each of the three sizes of the preferred range will be ordered.

The preferred range may comprise two, four or five sizes rather than three.

It will be readily understood that the invention may extend to apparatuses having four compressors, five compressors or 6 compressors in parallel.

In all cases, even if the N compressors are chosen from the predefined group of n sizes, it is possible for at least one other compressor that compresses the air to the outlet pressure of the N compressors or to a lower or higher pressure to supply the cold box or one of the cold boxes, this at least other compressor not being chosen the predefined group of sizes.

In all cases, each of the N compressors is capable of compressing air from ambient pressure to a first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3 and the total power of the N compressors being greater than 10 MW.

In all cases, for a given frame size, all the dimensions of the frames of the same size will be identical.

With this invention it becomes possible to design a series of air separation apparatuses such as those illustrated in the figures. In order to design numerous different apparatuses, for example from 2 to 15 apparatuses, use will be made only of the compressors of a range n of frame sizes. The flow rate of air separated in one apparatus may differ by at least 10% or even by at least 25% from that separated in the other apparatuses. That allows a great many compressors of the same size to be procured and then allows the spare parts to be stored in a centralized place, allowing for better stock control and reduced investment.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-14. (canceled)
 15. A method for producing an apparatus for separating air by cryogenic distillation, in which at least one gas from the air is obtained by way of product by separating air in a cold box comprising at least one column, the cold box being supplied with air at a first pressure by N air compressors compressing air from ambient pressure to the first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of the N compressors being greater than 10 MW, a group of n compressor frame size classes being predefined, n being equal to 2 or 3 or 4 or 5, the compressors in each class each having a frame of the same dimensions and therefore of the same size and the N compressors being chosen so that all the N compressors are of at least one size chosen from the n frame sizes of the group.
 16. The method as claimed in claim 15, in which the N compressors have the same frame size.
 17. The method as claimed in claim 15, in which at least a first of the N compressors has a frame of a first of the n sizes and at least a second of the N compressors has a frame of a second of the n sizes.
 18. The method as claimed in claim 17, in which at least a third of the N compressors has a frame of a third of the n sizes.
 19. The method as claimed in claim 15, in which the group of the n compressor frame sizes is made up of a first size for a compressor of a power of between 5 and 7 MW, of a second size, for a compressor of a power of between 12 and 16 MW, and of a third size for a compressor of between 19 and 25 MW.
 20. The method as claimed in claim 15, in which two compressors having the same frame size are adapted to handle different flow rates of compressed gas or gas to be compressed.
 21. The method as claimed in claim 15, in which two compressors having the same frame size have a different inlet pressure and/or outlet pressure.
 22. An installation for separating air by cryogenic distillation, comprising at least one cold box containing at least one column for producing at least one gas of the air by air separation, N air compressors, the installation comprising no air booster compressor apart from, potentially, an air booster compressor driven by an air turbine intended for the cold box or for at least one of the cold boxes or by a turbine driven by nitrogen coming from the cold box or from at least one of the cold boxes, and means for sending air from the N compressors to the cold box or to the cold boxes, each of the N compressors being capable of compressing air from ambient pressure to a first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of N compressors being greater than 10 MW, a group of n compressor frame sizes being predefined, n being equal to 2 or 3 or 4 or 5, each frame of a given size having the same dimensions and the N compressors being chosen so that all the N compressors have a frame chosen from the group of n sizes.
 23. The installation as claimed in claim 22, comprising only N air compressors.
 24. The installation as claimed in claim 22, comprising just one single cold box.
 25. A method for producing several apparatuses for separating air by cryogenic distillation, in each of which at least one gas from the air is obtained by way of product by separating air in a cold box comprising at least one column, the cold box being supplied with air at a first pressure by N air compressors compressing air from ambient pressure to the first pressure, the first pressure being greater than 12 bar abs and less than 35 bar abs, N being equal to or greater than 3, and the total power of the N compressors being greater than 10 MW, a group of n compressor frame size classes being predefined, n being equal to 2 or 3 or 4 or 5, the compressors in each class each having a frame of the same dimensions and therefore of the same size and the N compressors for each of the apparatuses being chosen so that all the N compressors are of one size chosen from the n frame sizes of the group.
 26. The method as claimed in claim 25, in which the separation apparatuses are not identical.
 27. The method as claimed in claim 25, in which each of the separation apparatuses is designed to separate a flow rate of air that differs by at least 10% from the flow rates of air which are separated in the other apparatuses.
 28. The method as claimed in claim 25, in which each of the separation apparatuses is designed to separate a flow rate of air that differs by at least 25% from the flow rates of air which are separated in the other apparatuses.
 29. The method as claimed in claim 25, in which the spare parts for the N compressors of each apparatus are stored in a common place. 